The present invention relates to a polymeric thickener for lubricating grease compositions, comprising a high molecular weight component and a low molecular weight component.
US-A-3.850.828 describes a lubricant grease composition, which is thickened with a polymeric mixture, comprising (1) a polyethylene with a molecular weight of 20,000-500,000, more preferably 50,000-250,000 and preferred polymer density above 0,94 gm/cc, and (2) an atactic polypropylene with a molecular weight preferable below 100,000 and a melt index above 20, preferably above 50. The ratio of the atactic polypropylene to the polyethylene is preferably 1:1 to 1:10, more preferably 2:1 to 5:1.
U.S. Pat. No. 2,917,458 describes a grease composition comprising an oil soluble amorphous polypropylene base having a molecular weight in the range of 300-10,000 and an intrinsic viscosity up to 0.4, 2 to 5 wt. % of an isotactic polypropylene having a molecular weight in the range of 100,000 to 1,000,000 and a melting point in the range of 250.degree. to 410.degree. F., and 5 to 35 wt. % of a soap-salt thickener.
According to this application "( . . . ) high molecular weight isotactic polypropylenes can be satisfactorily incorporated in a grease containing mixed salts of high and low molecular weight organic acids, through the agency of an oil soluble amorphous polypropylene of select molecular weight ( . . . ). It has now been found that by the use of amorphous polypropylene of select molecular weight, the isotactic polypropylene is successfully made to blend with the oil."
Therefore, according to this reference, the low molecular weight amorphous polypropylene is dissolved in the oil and increases the compatibility of the oil with the isotactic polypropylene thickener. Also, according to this reference the grease always contains a conventional soap-salt thickener.
According to the present application the low molecular weight component is not dissolved in the oil, but forms part of the polymer thickener, as can be seen from the viscosity of the grease obtained.
U.S. Pat. No. 3,290,244 describes a grease composition comprising a mineral lubricating oil, a thickening agent, and an oil soluble atactic homopolymer of polypropylene having a molecular weight in the range of 10,000-50,000 or an oil soluble atactic copolymer of ethylene and propylene having an intrinsic viscosity in the range of 0.3 to 4.0.
As a thickener, conventional thickeners such as fatty acid metallic soaps, inorganic thickeners such a colloids, silica and bentonite clay, etc. can be used in amounts of 5 to 40%.
According to this reference, the oil soluble atactic propylene polymer is dissolved in the oil present in the grease and serves to improve adhesion and cohesion. The use of a high molecular weight/low molecular weight polymeric thickener is neither described nor suggested.
Also, all the above references are silent with respect to the oil bleeding characteristics and/or the noise characteristics of the grease compositions obtained.
U.S. Pat. No. 3,392,119 describes a grease comprising a white mineral oil that has been thickened by the use of an ethylene-copolymer with a density of 25.degree. of at least 0.4 g/cm.sup.3 and a polypropylene homopolymer with a density of 25.degree. C. of between 0.890 and 9.20 g/cm.sup.3, the polyethylene to polypropylene weight ratio generally being in the range from about 10:1 to 1:10, preferably 3:1 to about 1:2.
According to this application "it was unexpectedly found that not only do white oils respond differently to such polyolefin thickening than do conventional lube oils, but the bleeding of the white oils with a polymer of ethylene and a polymer of polypropylene results in greases having improved non-bleeding characteristics", i.e. with reduced bleeding of the oil. According to the present application a grease with improved bleeding of the oil at low temperatures is obtained. Although it is possible to employ a white mineral oil according to US-A-3,392,119 in the present invention it is less preferred, because it will generally lead to an inferior mechanical stability compared to the preferred embodiments described hereinbelow.
Also, this reference is again silent with respect to the noise characteristics of the grease composition.